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Effect of hydraulic retention time and substrate availability in denitrifying bioelectrochemical systems
Narcis Pous, Sebastià Puig, M. Dolors Balaguer, Jesús Colprim
KI-Zusammenfassung
82 % KonfidenzThis study demonstrates that decreasing hydraulic retention time (HRT) in a tubular denitrifying bioelectrochemical system dramatically increases nitrate removal rates from 85 g N mNCC⁻³ d⁻¹ at 9.6 h to 849 g N mNCC⁻³ d⁻¹ at 0.46 h, with no nitrite accumulation. The cathode was poised at −0.320 V vs. Ag/AgCl. Critically, the improvement is driven by hydrodynamics and water flux rather than substrate availability alone, and N₂O emissions remain problematic (33–90% of removed NO₃⁻), suggesting the need for design modifications to suppress N₂O production.
Zentrale Erkenntnisse
Zielkonflikt Nitrate removal rate increased 10-fold (85 to 849 g N mNCC⁻³ d⁻¹) by decreasing HRT from 9.6 to 0.46 h, but removal efficiency dropped from 100% to 50%, revealing a trade-off between throughput and conversion.
Was heißt das. Operators must choose between high removal rates (low HRT, cascaded reactors) or high conversion (high HRT, single reactor); series operation is recommended to achieve both.
Schwellenwert At low HRTs (1.2–1.6 h), nitrate removal rate plateaued at 730–840 g N mNCC⁻³ d⁻¹ above 43.9 mg N L⁻¹ influent NO₃⁻, but at high HRT (3.4 h), activity sharply declined above 40.6 mg N L⁻¹, indicating substrate inhibition at longer residence times.
Was heißt das. Do not operate denitrifying BES at high HRTs with high influent nitrate; low HRT operation is more robust to substrate concentration variation.
Mechanismus System response to HRT change occurred in <0.25 days despite autotrophic biomass (slow growth ~1 d⁻¹), indicating activity modulation rather than growth, and hydrodynamics—not nitrate availability—is the dominant performance driver.
Was heißt das. Optimize reactor hydrodynamics and flow distribution rather than relying on substrate concentration to improve denitrification rates.
Leistung N₂O removal rate increased from 39–109 g N mNCC⁻³ d⁻¹ at high HRT to 436–466 g N mNCC⁻³ d⁻¹ at low HRT, but N₂O emissions (as % of removed NO₃⁻) remained high (33–90%), indicating incomplete reduction to N₂.
Was heißt das. Low HRT improves N₂O consumption kinetics but does not eliminate N₂O slip; alternative cathode designs or potentials may be needed to suppress N₂O formation.
Zielkonflikt Coulombic efficiency ranged 53–127% across HRTs, with >100% at low HRT indicating excess current supply relative to observed denitrification, suggesting heterotrophic activity or measurement uncertainty at high HRTs.
Was heißt das. At low HRTs, current supply exceeds denitrification demand; consider current control or potential adjustment to improve energy efficiency.
Ansatzpunkte6
Konkrete Lücken in dieser Publikation, jeweils mit einem Experiment, das sie schließen würde.
Hoch N₂O formation and reduction mechanism under varying cathode potentials (−0.320 V was fixed); test range −0.1 to −0.5 V vs. Ag/AgCl to identify potential window minimizing N₂O slip.
N₂O emissions remain 33–90% of removed NO₃⁻ across all HRTs; authors note this is higher than prior work but do not systematically vary potential to suppress N₂O formation.
Versuch. Potentiostatic sweep at fixed HRT (0.46 h) from −0.1 to −0.5 V vs. Ag/AgCl; measure NO₃⁻, NO₂⁻, N₂O, and current density.
Hoch Effect of internal recirculation or improved hydrodynamics on N₂O reduction; authors hypothesize lack of recirculation drives high N₂O emissions but do not test it.
Authors state: 'The difference in N₂O emissions could be explained through the different reactor design used' (lack of internal recirculation); no experimental validation provided.
Versuch. Retrofit tubular reactor with internal recirculation loop; compare N₂O emissions and removal rates at same HRT (0.46 h) and potential (−0.320 V).
Mittel Influent NO₃⁻ concentration range above 112 mg N L⁻¹ to determine if inhibition at high HRT (3.4 h) continues or reverses; maximum tested was 112 mg N L⁻¹.
At HRT 3.4 h, activity declined sharply above 40.6 mg N L⁻¹ (Fig. 5); mechanism unclear (FNA ruled out, no nitrite accumulation); testing higher concentrations may reveal saturation or true inhibition.
Versuch. HRT 3.4 h, influent NO₃⁻ 112–200 mg N L⁻¹; measure NO₃⁻ removal rate, NO₂⁻, and pH to assess FNA formation.
Mittel Microbial community composition and gene expression (nir, nor, nos genes) across HRT range to link hydrodynamic changes to N₂O reduction enzyme activity.
Authors conclude hydrodynamics dominate performance but do not characterize the microbial response; fast response (<0.25 d) suggests activity modulation, but mechanism is unexplored.
Versuch. 16S rRNA and metatranscriptomics at HRT 10.89, 1.2, and 0.46 h; correlate denitrification gene expression with N₂O removal rate.
Mittel Series operation of compact reactors (as recommended for scaling) not experimentally tested; only single-reactor performance evaluated.
Authors state: 'an appropriate approximation of scaling-up denitrifying BES would be the implementation of compact reactors connected in series' but provide no data on series performance, inter-reactor NO₃⁻/NO₂⁻ dynamics, or overall conversion.
Versuch. Two or three tubular reactors in series at HRT 0.46 h per reactor; measure NO₃⁻, NO₂⁻, N₂O at each outlet; compare overall conversion and N₂O emissions to single reactor.
Mittel Dissolved oxygen and pH in cathode compartment not monitored; only bulk effluent pH and conductivity reported; local conditions may drive N₂O formation.
Denitrification intermediates (NO₂⁻, N₂O) are sensitive to local pH and DO; no data on cathode microenvironment; could explain high N₂O emissions.
Versuch. Insert pH and DO microsensors in cathode at different radial/axial positions during HRT 0.46 h operation; correlate local conditions with N₂O production.
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Was gemacht wurde
- Systemtyp
- MERC
Was funktioniert hat
Zusammenfassung
This paper evaluates the influence of HRT and nitrate content on denitrifying BES.
Schlagwörter
Kennungen
- Zeitschrift
- Environmental Science: Water Research & Technology
- Jahr
- 2017
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